1. Field of the Invention
The present invention relates to a fluid dynamic bearing mechanism for a motor, in particular, a fluid dynamic bearing mechanism suitable for motors such as small and thin spindle motors, that have an excellent bearing rigidity and rotation accuracy, can securely prevent the rotor member from dislodging under external vibrations and shocks and allows for verification of the amount of lubricant being supplied.
2. Description of Related Art
Hard disk drives in recent years have become smaller and thinner, and begun running faster with the increasing memory capacity of hard disks. Consequently, the fluid dynamic bearing mechanism is required to be more compact and thinner as well as to have higher bearing rigidity, rotating accuracy, and reliability. Therefore, various kinds of improvements have been made in order to respond to those requirements.
For example, Publication of Unexamined Japanese Patent Application 2002-266878 discloses a fluid dynamic bearing mechanism for a motor having a lubricant being continuously supplied into a minute gap formed between a shaft member and a bearing member. A fluid seal part is formed to constitute a capillary structure on at least one end of a lubricant supply part in said minute gap for preventing leakage of said lubricant to outside. A rotating hub is attached on said shaft member at a location outside of said fluid seal part. An annular member is fitted on the shaft member at a location corresponding to the fluid seal part. An extending surface extending in the radial direction as well as an outer peripheral surface continuing axially from said extending surface and the outer edge of said extending surface on one end side in the axial direction of said annular member are arranged close to and facing against an extending surface extending in the radial direction. An inner peripheral surface continuing axially from said extending surface and the outer edge of said extending surface form said fluid sealing part. An extending surface extending in the radial direction on the other end side in the axial direction of said annular member abuts an end surface of the rotating hub to support the rotating hub in the axial direction. A thrust plate is fitted on the shaft member at its end opposite to the position corresponding to the fluid seal part.
Because of its constitution as described above, the bearing disclosed in the above publication has increased lubricant retaining space both in the radial and axial directions due to its fluid sealing part. Moreover, it is possible to view the amount of lubricant filling immediately after filling lubricant through this fluid sealing part, so that it is easier to adjust the amount of filling. Since the extending surface extending in the radial direction on the other end of the annular member in the axial direction abuts an end of the rotating hub, the rotating hub is supported axially so that its resistance against dislodgment as well as ease of machining and assembling are improved.
Furthermore, the invention disclosed by said publication forms a fluid dynamic thrust bearing by virtue of the lubricant filled in the minute gaps formed between both axial end surfaces of the thrust plate and the inner surface of the counter plate. The thrust plate also preventing the entire rotating part including the shaft member and the rotating hub from dislodging from the bearing member.
However, the invention disclosed by said publication is constituted in such a way that said fluid dynamic bearing mechanism cannot be made thinner due to the presence of the thrust plate fitted on the shaft member at the end part opposite to the location corresponding to the lubricant sealing part. Additionally, the axial length of the radial dynamic pressure bearing part is also shortened thus leaving some room for improvement in terms of achieving higher bearing rigidity and rotating accuracy on smaller and thinner fluid dynamic bearing mechanisms.
U.S. Pat. No. 3,155,529 discloses a fluid dynamic bearing mechanism having a thrust dynamic bearing formed by filling lubricant in a minute gap formed between the bottom surface of the upper wall of a rotor hub (rotating hub) and a cylindrical supporting member (bearing sleeve). The shaft is prevented from dislodgment by a ring-shaped member, which is fitted on the tip of the shaft, mating with an annular groove formed on a hollow part (bearing hole) of a supporting member at a location opposing the tip of the shaft.
However, since the bottom surface of the upper wall of the rotor hub is formed as one with the slide surface that constitutes the direct thrust dynamic pressure bearing in this invention, the entire upper wall of the rotor hub needs to be build solidly, thus making it impossible to make the rotor hub thinner in this invention. Moreover, although the thrust plate is omitted, the ring-shaped member provided on the tip of the shaft still prevents the fluid bearing from being made smaller and thinner.
Unexamined Japanese Patent Application 2001-103723 discloses a fluid dynamic bearing for a motor consisting of a stator assembly and a rotor assembly supported rotatably by a radial bearing containing lubricant. An air induction hole is provided in said rotor assembly to allow the outside air to be induced into a negative pressure region generated between said rotor and stator assemblies due to the rotation of the rotor assembly. The air induction hole here is so constituted as to allow it to be used as a hole for filling the lubricant into the fluid dynamic bearing part as well. Moreover, a stop ring is fitted on the tip of the rotating shaft in order to prevent the rotor assembly from dislodging.
Since the bearing disclosed in the above publication can alleviate the negative pressure in the negative pressure region formed between the stator assembly and the rotor assembly when the motor, which is designed flat and thin, is running at a high speed by inducing the outside air into the negative pressure region through the air induction hole, it prevents possible problems such as leakage of the lubricant from the bearing, etc. The air induction hole can also be used as the lubricant filling hole, thereby simplifying the lubricant replenishing operation, which contributes to productivity and realization of a longer motor life. Moreover, the stop ring engages with the lower end of the radial bearing (bearing sleeve), preventing the rotor assembly from dislodging from the stator assembly.
However, the invention disclosed in this publication has no means of increasing the lubricant retaining space. The invention is also indifferent to the prevention of the rotor hub that constitutes the rotor assembly from dislodging as well as to the ease of machining and assembling of components. Furthermore, since the thrust bearing is protruding outward from the frame's constraint (base member), there is still room for improvement in terms of making the fluid dynamic bearing mechanism thinner and flatter.